The present invention relates to a projection lens system for use in a rear type projection television, and more particularly to a projection lens system capable of reducing the number of lenses in the system and correcting chromatic aberrations.
Generally, a projection television enlarges and projects lights of red (R), green (G) and blue (B) emitted from a cathode ray tube (CRT) onto a CRT screen through the use of a plurality of projection lenses. Generally, the projection methods are classified into the following: a front type projection method in which the images are projected onto the front surface of a CRT screen such that the images reflected from the CRT screen can be viewed by a user, and a rear type projection method in which the images are projected onto the rear surface of the CRT screen such that the images transmitted through the CRT screen can be viewed by the user.
In rear type projection televisions, which use the rear type projection method, since the lights entering the screen are not reflected from the screen but transmitted through it, definitive images can be provided even in the bright peripheral areas of the screen. Accordingly, rear type projection televisions are generally preferred by users. However, such projection televisions require wider field-angles in order to enlarge projection magnification. At the same time, it is desirable to reduce the depth of the projection system as much as possible. Also, the rear-type projection televisions require smaller brightness values for the lens system (F-numbers, which are equal to an aperture distance divided by an effective focal distance, can be used to quantify brightness values) in order to form a bright image on the screen. Furthermore, light quantity differences between the center of the screen and the periphery thereof should be as little as possible. Finally, chromatic aberrations generated due to differences between the R, G and B lights need to be corrected.
To meet the above requirements, several projection lens systems have been developed. However, in conventional projection lens systems, a lens which is located in the center of the lens system provides most of the optical power in the lens system. Here, to represent the magnitude of the optical power, when the light is converged into a focal point, the degree of convergence is expressed as a positive value, while when the light is diverged from the light source, the degree of divergence is expressed as a negative value. When a single lens provides most of the entire optical power, the image can be made relatively bright. However, since a peripheral light quantity ratio, (that is, a light quantity ratio of a peripheral portion with respect to the screen center portion) is thereby rendered low (for example less than 21%), the periphery of the screen is relatively dark. Also, since correction of the chromatic aberrations is often not sufficient, it is difficult to form an excellent image. Further, since the glass lenses utilized are designed to have spherical surfaces at least six lenses have been needed to correct for aberration.